1. Field of the Invention
The present invention relates generally to a device for measuring and showing a physical quantity such as the depth of water (in terms of water pressure), the altitude of a flying object (in terms of atmospheric pressure) or other physical quantities by means of a pointer moving around a numbered face, and particularly to such a gauge using a pulse motor of reversible type for driving an associated pointer.
Japanese Patent Application Laid-Open Nos. 56-19480 and 58-193466 show a digital type water depth gauge in which an analogue value of a water pressure provided by a water-pressure sensor is converted to a corresponding digital value, and then the digital value is shown by an electro-optical display device appropriate for the purpose.
In case that a liquid-crystal display device is used, the device advantageously requires a relatively small amount of electric power for driving the same, but disadvantageously the display presented thereby is hardly visible at a water depth where the light is dim. In case that a light emitting diode or any other light emitting display device is used, advantageously the device can present a visible in the dark in water, but disadvantageously it requires a relatively large amount of electric power for operation.
Still disadvantageously the degree of freedom with which the appearance of the display device is designed is limited if a liquid crystal display, a light emitting diode or any other electro-optical digital display device is used. For instance, if it is desired that a watch for diver's use is equipped with a water depth gauge, the use of an analogue type gauge using a pointer moving around a numbered face is, in fact, effective to expand horizon in ornamental designing. There are, however, problems as to how a pointer is driven to follow the varying water depth. Assume that use is made of a pointer-driving system as found in a voltmeter. Then, an electric current would have to be continuously supplied to an associated coil to show the water depth all the time. This requires too large an amount of electric power for a wrist watch using a minute cell to supply to a water depth gauge. An attempt to use an intermittent driving type electric-to-mechanical converter or a pulse motor in the hope of taking advantage of less electric power consumption, however, has been deadended because of the lack of control means appropriate for permitting the pointer to follow the varying water depth. Thus, there has been no analogue type water depth gauge among those which use electronic devices for measuring and indicating the depth in water.